Similar to humans, companion animals such as dogs and cats can become obese. The result of excessive accumulation of adipose tissue (body fat) results in animals being overweight or obese.
Adipose tissue is an important energy depot. This is important for the survival of wild animals because their daily food supply is limited and uncertain. However, the body fat level in wild animals is much lower than that of domesticated animals.
It has been estimated that 25-44% of domesticated dogs and cats are overweight or obese in the United States and Europe (Hand, M S, Armstrong P J, Allen T A. Obesity: Occurrence, treatment, and prevention. Vet Clin North Am Small Anim Pract. 1989, 19:447-474; Scarlett J M, Donoghue S, Saidla J, Wills J. Overweight cats: prevalence and risk factors. Int. J. Obes. 1994, 18:S22-S28). Dogs that are overweight or obese have health problems, especially in middle age (i.e., dogs that are between 6-8 years old). If these health problems caused by being overweight are not corrected, the risks increase for the development of a number of chronic diseases, including diabetes mellitus, cancer, hypertension, pulmonary, cardiovascular, and degenerative joint disease. In addition, a dog may suffer from skin problems, reduced resistance to infectious diseases, and increased mortality. Increased oxidative stress is, at least partially, believed to be responsible for the increased risk of the above-mentioned diseases associated with obesity. Further, it is well known that obesity places stress on an animal's body.
In addition to its role as an energy surplus storage site, adipose tissue is an endocrine organ. The endocrine system controls a variety of functions in an animal's body through hormones and cytokines. The homeostasis of these hormones is disturbed in obesity.
Leptin is a protein hormone that functions to regulate appetite, metabolism and sexual maturation. Leptin is synthesized in fat cells (adipocytes) and secreted into the blood stream. Therefore, serum levels of leptin correlate with the amount of body fat. Leptin concentrations increase during overfeeding or weight gain and decrease during fasting or weight loss. It has been shown in humans that leptin highly correlates to how much fat is stored in a body, with greater levels found in individuals with more fat and reduced levels in those who dieted. Similarly, it has been shown that in obese dogs, the plasma concentration of leptin was increased regardless of the dogs' breed, age or sex.
It is known that leptin's effects on body weight are mediated through its effects on hypothalamic centers that control body temperature, energy expenditure, as well food intake. It has been shown that other hormones or endocrine substances involved in body weight regulation or its distribution, such as catecholamines, corticosteroids, insulin, sex hormones, and growth hormone, can be triggered by leptin.
Certain pro-inflammatory cytokines are synthesized in adipose tissue. For example, Tumor Necrosis Factor-α (TNF-α) is synthesized in adipocytes and acts locally. In normal animals, TNF-α regulates the number of adipocytes , reduces fat accumulation and promotes fat breakdown. TNF-α also stimulates leptin synthesis and secretion from adipocytes. TNF-α inhibits insulin action; increased TNF-α leads to insulin resistance.
Interleukin-6 (IL-6) is another pro-inflammatory cytokine associated with adipose tissue. In normal animals, IL-6 reduces fat accumulation, suppresses appetite and increases fat breakdown. IL-6 stimulates the secretion of acute-phase proteins (e.g., C-reactive protein) by the liver. Synthesis and secretion of IL-6 by adipocytes may be one of the major sources of circulating IL-6. Circulating IL-6 concentrations are strongly correlated with adiposity.
Excess adipose tissue results in a hyperactive endocrine adipose organ, which, in turn, affects other endocrine systems (e.g., insulin etc), metabolic functions, oxidative stress level, and inflammation.
Increased oxidative stress has been linked with obesity. Isoprostanes are a stable in vivo marker of oxidative stress and tissue damage (Lynch S M, Morrow J D, Roberts II L J, Frei B. Formation of non-cyclooxygenasae-derived prostanoids (F2-isoprostanes) in plasma and low density lipoprotein exposed to oxidative stress in vitro. J. Clin. Invest. 1994, 93: 998-1004; Morrow J D, Hill K E, Burk R F, Nammour T M, Badr K F, Roberts II L J, A series of prostaglandin F2-like compounds are produced in vivo in humans by a non-cyclooxygenase, free radical-catalyzed mechanism, Proc. Natl. Acad. Sci. 1990, 87: 9383-9387). Isoprostanes are produced in vivo by oxidative damage of arachidonic acid in cell membrane phospholipids and lipoproteins in the blood. They are a chemically stable end-product of lipid peroxidation. Isoprostanes released by phospholipases from membrane phospholipids and lipoprotein particles circulate in the plasma and are excreted in urine. Higher levels of isoprostanes in the blood and urine mean higher oxidative stress and tissue damage in vivo.
Obese men have been found to have significantly higher plasma concentrations of isoprostanes than nonobese men (P<0.05). The plasma levels of isoprostanes were significantly correlated with body mass index (r=0.408; P<0.05), body fat weight (r=0.467; P<0.05), visceral (r=0.387; P<0.05) and total fat area (r=0.359; P<0.05) in all (obese and nonobese) men. (Urakawa H, Katsuki A, Sumida Y, Gabazza E C, Murashima S, Morioka K, Maruyama N, Kitagawa N, Tanaka T, Hori Y, Nakatani K, Yano Y, Adachi Y, Oxidative stress is associated with adiposity and insulin resistance in men. J Clin Endocrinol Metab, October 2003; 88(10):4673-6).
Obesity per se can lead directly to hypertension. This has been shown in dogs in which obesity was induced by feeding them a high-fat diet. It has been shown that weight gain in the dog is associated with an increase in blood pressure, heart rate, cardiac output, and glomerular filtration rate. In addition, these dogs showed a progressive alternation in cardiac diastolic function. Angiotensinogen is released into the blood where it serves as a precursor for two proteins: angiotensin I and angiotensin II. The actions of these proteins lead to an increase in blood pressure through the reabsorption of sodium in the kidney.
Animals accumulate fat by eating more calories than are expended as energy. If the intake of energy exceeds its expenditure, fat accumulates. If fat is to be removed from the body, fewer calories must be consumed or more calories must be used than consumed. Physical activity changes the expenditure of energy. For example, a marked decrease in physical activity may lead to obesity. Physical inactivity restricts energy expenditure and may contribute to increased food intake. The basal metabolic rate (BMR) is the energy expended by an animal in a resting state and represents the energy required to perform normal body functions.
Excessive accumulation of body fat in animals results in excessive weight gain and obesity. This occurs when fat synthesis exceeds fat breakdown. Substances that inhibit fat synthesis and/or promote fat breakdown may be used to either reduce or prevent excessive weight gain or obesity in animals. Current weight loss diets result in loss of both body fat and lean body mass. In addition, current weight loss diets are not designed to reduce the oxidative damage caused by obesity.
An additional risk factor for obesity in animals is removal of the sex organs. Procedures such as neutering, spaying, ovariectomy, castration, and the like, are frequently performed on animals for population control. However, weight gain is commonly observed in animals following these procedures. (Harper E J, Stack D M, Watson T D, Moxham G. Effects of feeding regimens on bodyweight, composition and condition score in cats following ovariohysterectomy. J Small Anim Pract. 2001, 42:433-438; Robertson I D. The association of exercise, diet and other factors with owner-perceived obesity in privately owned dogs from metropolitan Perth, W A. Prev Vet Med. 2003, 58:75-83.) It is believed that the weight gain is a consequence of the diminished production of sex hormones by the sex organs and markedly decreased levels of endogenous sex hormones following the removal of the sex organs.
Among the sex hormones, estrogens and androgens have been found to play a role in the metabolism of adipose tissue. Indeed, diminished levels of estrogen and testosterone have been correlated with increased accumulation of body fat. (Pergola G D, The adipose tissue metabolism: Role of testosterone and dehydroepiandrosterone. Int. J. Obesity, 2000, 24: S59-S63; Cooke P S and Naaz A. Role of Estrogens in Adipocyte Development and Function. Exp Biol Med. 2004, 229:1127-1135; Mohamed, M K et al., Effect of long-term ovariectomy and estrogen replacement on the expression of estrogen receptor gene in female rats. Eur. J. Endocrinol. 2000, 142:307-314.) The sex hormones may affect adipose tissue in different ways, for example, by affecting the number and size of adipocytes, lipogenesis, and lipolysis, modulating appetite or energy expenditure, and the like. (Pergola G D, The adipose tissue metabolism: Role of testosterone and dehydroepiandrosterone. Int. J. Obesity, 2000, 24: S59-S63; Cooke P S and Naaz A. Role of Estrogens in Adipocyte Development and Function. Exp Biol Med. 2004, 229:1127-1135; Naaz, A et al. The soy isoflavone genistein decreases adipose deposition in mice. Endocrinol. 2003, 144:3315-3320). In this regard, hormone replacement therapy and dietary supplementation have been studied as a means to reverse these effects. (Sayegh, R A et al. Impact of hormone replacement therapy on the body mass and fat compositions of menopausal women: a cross-sectional study. Menopause. 1999, 6:312-315; Blathena S J et al. Beneficial role of dietary phytoestrogens in obesity and diabetes. Am. J. Clin. Nutr. 2002, 76:1191-1201).
With respect to dietary supplementation, mounting evidence suggests that phytoestrogens may play a role in enhancing lipid metabolism and diminishing deposition of adipose tissue. (Naaz, A et al. 2003; Bhathena, S J et al. 2002; and Wagner J D, et al. Soy protein with isoflavones, but not an isoflavone-rich supplement, improves arterial low-density lipoprotein metabolism and atherogenesis. Arterioscler Thromb Vasc Biol. 2003, 23:2241-2246).
Phytoestrogens are chemicals produced by plants that have a similar structure to mammalian estrogens (Clarkson T B, Anthony, M S, Morgan T M. Inhibition of postmenopausal atherosclerosis progression: a comparison of the effects of conjugated equine estrogens and soy phytoestrogens. J Clin Endocrinol Metab. 2001, 86:41-47), and are capable of interacting with the estrogen receptor on adipose tissue in many animal species including humans, rats, monkeys, and mice. (Naaz, A et al. 2003, supra; Linford N J, and Dorsa D M. 17 beta-Estradiol and the phytoestrogen genistein attenuate neuronal apoptosis induced by the endoplasmic reticulum calcium-ATPase inhibitor thapsigargin. Steroids. 2002, 67:1029-1040). Phytoestrogens are subdivided into three major classifications, coumestans, lignans, and isoflavones. Isoflavones have demonstrated significant positive effects with respect to reducing adipose deposition, reducing serum low density lipoproteins, inhibiting atherosclerosis, and the like, in subjects to which they were administered. (Bhathena, S J, et al. 2002; Naaz, A, et al. 2003; Wagner J D 2003; Kawakami Y et al. Regulative actions of dietary soy isoflavone on biological antioxidative system and lipid metabolism in rats. J. Agric. Food. Chem. 2004, 52:1764-1768; and Fang Y C et al. Effect of genistein supplementation on tissue genistein and lipid peroxidation of serum, liver and low-density lipoprotein in hamsters. 2004, 15:142-148).
Despite the positive effects observed with dietary supplementation in humans and rodents, there still exists a need to produce food or medicinal formulations for administration to companion animals, and even more so for administration to companion animals that have undergone a procedure to remove sex organs such as spaying or neutering. Ideally, such food or medicinal formulations, and the methods to use them, would facilitate the loss of body fat, minimize loss of lean body mass, and reduced oxidative stress damage in the animals.